This invention relates to a surge absorbing circuit suitable for an electronic device of communication equipment such as telephone sets, facsimiles, telephone switch-boards, modems, and the like and to a thermal response switch used for such surge absorbing circuit. More particularly, it relates to a surge absorbing circuit capable of protecting electronic devices from continuous overvoltages or overcurrents and to the thermal response switch used therefor.
It is known to connect a surge absorbing element to a pair of input lines of an electronic device in parallel with the electronic device, the surge absorbing element is designed to operate at a higher voltage than the operating voltage of the electronic device. Such a surge absorbing element is a resistor having a high resistance value when the voltage applied thereto is lower than the discharge starting voltage thereof, but its resistance value is equal to as low as several tens of ohms or less when the voltage applied thereto is equal to or higher than the discharge starting voltage thereof. Accordingly, when surge voltages, such as, lightning surges, etc., are instantaneously applied to an electronic device, the surge absorbing element discharges to absorb the surge voltages, and serves to protect the electronic device from the surge voltages. Thus, when an overvoltage or overcurrent due to an accident is continuously applied to the electronic circuit including the electronic device, a certain amount of current continuously flows through the surge absorbing element. This results in the surge absorbing element being heated to high temperatures. The heat radiating from the surge absorbing element can cause the electronic equipment surrounding the surge absorbing element to catch fire.
While it does not usually happen that such accidental overvoltages or overcurrents are continuously applied to the circuit, it has recently become desirable in more fields to take the maximum safety measures to avoid such accidental problems. As an example, UL (Underwriter's Laboratories Inc.) of the U.S.A. prescribes a safety standard for surge absorbers so that they do not cause fire or electrical shock in communication equipment when continuous overvoltages or overcurrents are applied.
A known surge absorber (1) capable of preventing fires of the communication equipment due to continuous overvoltages or overcurrents includes a fuse or a lower melting point metallic member adhered on the surface of the surge absorber element, and the resultant fuse or lower melting point metallic member is connected in series with the surge absorbing element (Unexamined Published Japanese Patent Applications No. 63-11022 and 63-18923).
We have also disclosed a surge absorbing circuit (2) wherein a surge absorbing element is connected to a pair of input lines of an electronic device in parallel with the electronic device and having a thermal response switch which is open by heating and closed by cooling connected to one side of the input lines at an input side of the surge absorbing element (Japanese Patent Application No. 3-28066, corresponding to U.S. patent application Ser. No. 07/827,375, filed Jan. 29, 1992). The thermal response switch is provided in the vicinity of the surge absorbing element and uses a thermal response piece, such as, a bimetal as a movable contact point.
We have also disclosed a thermal response switch (3) comprising a conductive movable body, a pair of thermal response pieces for holding the movable body upon non-heating and releasing it upon non-heating, a spring for separating the released movable body from the thermal response piece, and a reset pin for restoring the separated movable body (Japanese Patent Application No. 3-188027, corresponding to U.S. patent application Ser. No. 07/906,272, filed Jun. 26, 1992).
However in surge absorber (1) in which the fuse or the low melting point metallic member is connected in series with the surge absorbing element, if the fuse or the low melting point metallic member blows due to an applied overvoltage or overcurrent, the surge absorbing circuit is in the "open state", and it is troublesome to replace the surge absorber with a new one. In particular, if the surge absorbing element together with the fuse or the like is covered with a housing, a problem arises because it may be difficult to visually check the melting state of the fuse.
A problem with surge absorbing circuit (2) is that after the circuit is in the "open state" with the overvoltages or overcurrents imposed thereon, an automatic restoration function is available when the applied overvoltages or overcurrents cease. However, the thermal response piece of the movable contact point has only a slight contact pressure with the fixed contact point piece. As a result, vibration of the unit or thermal response switch causes temporary separation of the thermal response piece from the fixed contact point piece, which, in turn, results in the circuit disadvantageously being in "open state".
In thermal response switch (3), while the movable body securely contacts the thermal response piece due to the spring bias of a pair of the thermal response pieces during non-heating, and thus avoids any problem due to vibration, the structure of the switch is complicated resulting in higher costs because of larger housing size.